Silicon-Oxide-Nitride-Oxide-Silicon (SONOS) memory is one type of non-volatile memory. An SONOS memory cell can be formed from a metal-oxide-semiconductor field-effect transistor (MOSFET) with an additional silicon nitride layer inserted in the transistor's gate oxide layer and thus has an Oxide-Nitride-Oxide (ONO) layer including a lower silicon oxide layer, a middle silicon nitride layer, and an upper oxide layer sequentially stacked on one another on a channel of the transistor. Since a silicon nitride layer contains a large number of charge trapping sites able to hold electrostatic charges, the conductivity of the channel can be regulated by an amount of electrons trapped in the silicon nitride layer of the ONO layer.
When a positive control signal is applied to a gate electrode of the transistor, electrons traveling between the source and drain regions of the transistor may tunnel through the lower oxide layer of the ONO layer and be trapped in the silicon nitride of the ONO layer, resulting in an energy barrier between the drain and source regions. Thus, a threshold voltage Vth of the transistor may be increased (e.g., programming). On the other side, when a negative control signal is applied to the gate electrode, the trapped electrons can be released to lower the threshold voltage Vth (e.g., erasing). After programming or erasing the SONOS memory cell, a controller of the SONOS memory can determine the state of the SONOS memory cell by applying a small voltage across the source and drain regions of the transistor. If the controller detects that current greater than a predetermined value flows through the source and drain electrodes, the controller determines that the SONOS memory cell is in a state without any trapped electrons (or with a small amount of trapped electrons but less than a reference number) in the silicon nitride layer, which is considered as, for example, a logical value “1.” If the controller detects that current less than the predetermined value flows through or no current flows through the source and drain electrodes, the controller determines that the SONOS memory cell is in a state with trapped electrons in the silicon nitride layer, which is considered as, for example, a logical value “0.”